1. Field of the Invention
The present invention relates to a radiation solid-state detector having a charge storing section which stores charges of a quantity corresponding to the dose of the projected radiation as latent image charges, and a method and device for using the detector to record radiation image information as a static latent image or read out the recorded static latent image.
2. Description of the Prior Art
To reduce the dose of exposure of the subject and improve the performance of diagnosis in medical radiation photographing, etc., a method (for example, U.S. Pat. No. 4,535,468, etc.) has been known up to now which uses, as a photosensitive material, a radiation solid-state detector (a static recorder) with a photoconductor, such as a selenium plate, responding to radiation, such as X-ray; projects the X-ray onto the detector, and stores the charges of the quantity corresponding to the dose of the projected radiation in the charge storing section in the detector for recording of radiation image information as a static latent image; and uses a laser beam or a line light source to scan the detector in which the radiation image information has been recorded, for reading out the radiation image information from the detector.
The method according to the above-mentioned U.S. Pat. No. 4,535,468 uses a three-layer structure detector which has an X-ray photoconductive layer, a charge storing layer (also called an intermediate layer or a trap layer) for storing of charges generated in the X-ray photoconductive layer, and a photoconductive layer for reading in this order, and in recording, applies a high voltage across the electrodes provided on both sides of the three layer structure, and projects X-ray to store latent image charges in the charge storing layer, and then short-circuits the electrodes to read out the latent image charges. With this method, the photoconductive layer for reading in the detector is thinner than the X-ray photoconductive layer in order to increase the reading speed for improving the response.
However, with the method according to the above-mentioned U.S. Pat. No. 4,535,468, the photoconductive layer for reading is thinner than the X-ray photoconductive layer, therefore, a problem results that the quantity of the signal charges detected from the outside is small. Further, the charge mobility is low for both electron and hole, and so, the charge storing layer cannot be thin. This is because increasing the charge mobility decreases the response speed, resulting in a residual image being produced. In other words, with this method, it is difficult to make the high speed response in reading compatible with efficient taking out of signal charge.
On the other hand, in Japanese Patent Application No. 10 (1998)-232824 and Japanese Patent Application No. 10 (1998)-271374, the present applicant has proposed a radiation solid-state detector which allows the high speed response in reading to be compatible with the efficient signal charge taking out, a recording device for recording of radiation image information in this detector, and a method and device for reading out the radiation image information from the detector in which the radiation image information has been recorded as a static latent image.
This method as stated in Japanese Patent Application No. 10 (1998)-232824, etc. uses a radiation solid-state detector provided with a photoconductive layer for recording which exhibits conductivity when irradiated with the radiation for recording or the light emitted by excitation on this radiation, a charge transporting layer which acts almost as an insulator for the latent image charges, while acting roughly as a conductor for transported charges having a polarity opposite to that of the latent image charges, and a photoconductive layer for reading which exhibits conductivity when irradiated with the electromagnetic wave for reading in this order. Radiation for recording is projected onto the photoconductive layer for recording in the detector, and by storing the charges of the quantity corresponding to the dose of the projected radiation in the charge storing section formed roughly at the boundary between the photoconductive layer for recording and the charge transporting layer, radiation image information is recorded as a static latent image. By reading out the static latent image, the radiation image information is provided.
The radiation solid-state detector according to the present invention is a radiation solid-state detector which further improves the detector, etc. as stated in the above-mentioned Japanese Patent Application No. 10 (1998)-232824, and Japanese Patent Application No. 10 (1998)-271374, i.e., a radiation solid-state detector which has a charge storing section for storing charges in a quantity corresponding to the dose of radiation which has been projected, and records radiation image information as a static latent image in the charge storing section,
wherein a first electrode layer having a permeability to radiation for recording or light emitted by excitation on the radiation, a photoconductive layer for recording which exhibits a conductivity when irradiated with the radiation for recording or the light, a photoconductive layer for reading which exhibits a conductivity when irradiated with an electromagnetic wave for reading, and a second electrode layer having a permeability to the electromagnetic wave for reading are provided in this order,
and a first conductive member for outputting an electric signal corresponding to the quantity of the latent image charges stored in the charge storing section formed between the photoconductive layer for recording and the photoconductive layer for reading is provided in the second electrode layer or between the first electrode layer and the second electrode layer.
The first conductive member may have any shape, but, it is preferable that the shape has no effect on the process of latent image forming (transferring and storing of the latent image charges) in recording, or the process of charge-recoupling of the latent image charges with charges opposite in polarity to the latent image charges, i.e., the transported charges. For example, when the first conductive member is provided in the photoconductive layer for recording or on the face of the photoconductive layer for recording which is for the photoconductive layer for reading, the shape is preferably that which will not hinder the latent image charges generated in the photoconductive layer for recording being transferred to the charge storing section. When the first conductive member is provided in the photoconductive layer for reading or in the later-described charge transporting layer or trap layer, the shape is preferably that which will not hinder the transported charges generated in the photoconductive layer for reading being transferred to the charge storing section. For this, it is recommended to take such a measure as providing holes having a desired shape, such as a circle and a square, in correspondence with the pixels, or providing an elongated rectangular hole extending along the direction of the pixel arrangement.
When this first conductive member is disposed in the photoconductive layer for recording, it is preferably permeable to the radiation for recording or the light emitted by excitation at the time of radiation, so that the radiation or the like can sufficiently get into the photoconductive layer for recording, and thus the charge generation process in the photoconductive layer is not affected.
To enhance the response in reading, the smaller the sum of the thickness of the layer forming the charge storing section and that of the photoconductive layer for reading for a given thickness of the photoconductive layer for recording, the better.
The electrode constituting the second electrode layer and/or the first conductive member of the radiation solid-state detector are preferably a stripe electrode comprising a number of linear electrodes.
The term xe2x80x9clinear electrodexe2x80x9d means an electrode having a long and slender shape as a whole, and so long as it has a long and slender shape, it may of any shape, such as columnar or prismatic, but it is preferable that the linear electrode be particularly a flat plate electrode. To prevent the latent image forming process and the charge-recoupling process from being affected as stated above, such a measure as providing the linear electrode with holes having a desired shape, such as a circle and a square, in correspondence with the pixels, or with an elongated rectangular hole extending along the direction may be taken.
Here, when the electrode constituting the second electrode layer and the first conductive member are a stripe electrode, it is preferable that the linear electrodes of the first conductive member be disposed so that they are opposed to or almost orthogonally intersect the linear electrodes of the electrode constituting the second electrode layer.
The phrase xe2x80x9care disposed so that they are opposed toxe2x80x9d means that each linear electrode of the first conductive member is located roughly just above the linear electrode of the electrode constituting the second electrode layer with a specified spacing, and disposed so that they are opposed to each other in the longitudinal direction. The phrase xe2x80x9care disposed so that they almost orthogonally intersectxe2x80x9d means that the linear electrodes of the first conductive member almost orthogonally pass over the linear electrodes of the electrode constituting the second electrode layer.
In this case, when the first conductive member is further disposed in the photoconductive layer for recording, the photoconductive layer for reading, or the charge transporting layer, it is recommended that the width of the linear electrode of the first conductive member be 5 to 30% of the pitch for the linear electrodes of the second photoconductive layer to prevent the latent image forming process in recording and the charge-recoupling process from being affected.
When the first conductive member and the electrode constituting the second electrode layer are linear electrodes, the linear electrodes of the first conductive member being disposed in the second electrode layer, it is preferable that each linear electrode of the first conductive member be disposed between linear electrodes of the second electrode layer so that they are in parallel with one another, and be not permeable to the electromagnetic wave for reading to prevent the electromagnetic wave from getting into the photoconductive layer for reading and deteriorating the reading resolution.
Further, with the radiation solid-state detector according to the present invention, it is preferable that a second conductive member for causing the latent image charges to have the same potential be discretely provided in the charge storing section for each pixel for an image represented by the electric signal. Particularly, it is recommended that the second conductive member be provided on the face (boundary) of the photoconductive layer for recording which is for the photoconductive layer for reading.
Here, the phrase xe2x80x9cbe provided for each pixelxe2x80x9d means that preferably one conductive member is provided for each pixel so that the charges around the pixel can be concentrated on the pixel central portion in reading by causing the latent image charges to have the same potential, and does not involve the style in which a number of conductive members are disposed at random for one pixel, so that when reading the charges around the pixel cannot be concentrated on the pixel central portion.
The word xe2x80x9cdiscretelyxe2x80x9d means that the respective conductive members are disposed in a state isolated from one another, i.e., in the floating state in which they are not connected to one another. When a plurality of conductive members are provided for one pixel, it is preferable that the members for one pixel be electrically connected to one another.
The size of this second conductive member is preferably set at a value approximately equal to the pixel pitch. Or, it may be set at a value smaller than the pixel pitch, say, less than half, and the second conductive member may be disposed in the pixel central portion to concentrate the latent image charges on the pixel central portion. The size of this second conductive member refers to the diameter for a circular conductive member, and the length of each side for a square conductive member. The shape of the conductive member may be any shape, such as a circle and a square.
When a detector with which the electrode constituting the second electrode layer and the first conductive member are a stripe electrode, and the linear electrodes constituting both electrodes are disposed so that they orthogonally intersect each other is used, the electric field is concentrated on the location where both linear electrodes cross. It is therefore preferable that, by disposing the second conductive member in correspondence with the location where both linear electrodes cross, the charge concentration efficiency be increased. In recording, the second conductive members may be left open.
The radiation solid-state detector according to the present invention may have a charge transporting layer, which acts roughly as an insulator for the latent image charges, and roughly as a conductor for charges opposite in polarity to the latent image charges between the photoconductive layer for recording and the photoconductive layer for reading, and the charge transporting layer may form the charge storing section. Alternatively, the radiation solid-state detector may have a trap layer for catching the latent image charges between the photoconductive layer for recording and the photoconductive layer for reading, the trap layer forming the charge storing section.
When the radiation solid-state detector is to have a charge transporting layer or a trap layer, the first conductive member may be provided at the boundary between the photoconductive layer for reading and the trap layer or at the boundary between the photoconductive layer for reading and the charge transporting layer, or in the charge transporting layer or in the trap layer. Further, the second conductive member may be provided at the boundary between the photoconductive layer for reading and the charge transporting layer, or at the boundary between the photoconductive layer for reading and the trap layer.
The radiation image recording method according to the present invention is a radiation image recording method which projects radiation onto the above-stated radiation solid-state detector to store the charges of the quantity corresponding to the dose of the projected radiation in the charge storing section of the radiation solid-state detector as latent image charges for recording of radiation image information as a static latent image in the charge storing section,
wherein a control voltage to adjust the electric field formed between both electrode layers by a DC voltage applied across the first electrode layer and the second electrode layer in the radiation solid-state detector is applied to the first conductive member.
The phrase xe2x80x9cprojects radiation onto the radiation solid-state detectorxe2x80x9d means that the radiation for recording which carries radiation image information about the subject is directly or indirectly projected onto the detector, and is not limited to the radiation for recording being directly projected onto the detector, meaning that, by projecting the radiation onto a scintillator (a fluorescent body), for example, the light, such as fluorescence, emitted by excitation on the radiation for recording is projected onto the detector.
The term xe2x80x9ccontrol voltagexe2x80x9d means a voltage of a magnitude with which the first conductive member can have a specified effect on the latent image charge storing process in recording, and it can be set at a voltage of a magnitude which provides roughly the same electric field as that which would be formed if the first conductive member were not provided.
Further, by positively bringing the control voltage close to or away from the potential at the second conductive layer, the region in which latent image charges are formed can be provided with a change. As a result, the signal taking-out efficiency and the reading response speed can be improved.
The control voltage may be either a DC voltage or an AC one. The AC voltage is not limited to the sinusoidal wave, and may be of any waveform, provided that it can improve the signal taking-out efficiency and the reading response speed.
The radiation image reading method is a radiation image reading method which, from the above-stated radiation solid-state detector in which radiation image information has been recorded as a static latent image, reads out the radiation image information, wherein the charges corresponding to the latent image charges stored in the charge storing section of the radiation solid-state detector are read out through the first conductive member to provide an electric signal at a level corresponding to the quantity of the latent image charges.
The phrase xe2x80x9cthe charges are read out through the first conductive memberxe2x80x9d means that the charges are read out at least through the first conductive member, and in addition to the current flowing between this first conductive member and the electrode of the second electrode layer, the current flowing between the electrode of the first electrode layer and the electrode of the second electrode layer may also be detected.
The electromagnetic wave for reading may be a continuous wave generated continually or a pulse wave generated in the form of a pulse, but, the pulse wave permits a higher current to be detected, allowing a pixel having a small quantity of latent image charges to be detected as a sufficiently high current, which means, it can substantially improve the S/N ratio, which is advantageous.
However, when the pulse wave is used with a detector with which the electrode constituting the second electrode layer and the first conductive member are a stripe electrode, and the linear electrodes constituting both electrodes are disposed so that they orthogonally intersect each other, the electric field is concentrated at the location where both linear electrodes cross, and the latent image charges are also concentrated at this location. It is therefor preferable that the electromagnetic wave for reading be projected onto the location in the photoconductive layer for reading which corresponds to this crossing location. When a detector with which a second conductive member is provided is used, the latent image charges are stored, being concentrated on the second conductive member, and it is therefore preferable that the electromagnetic wave for reading be projected at least onto the location in the photoconductive layer for reading which corresponds to the location where this second conductive member is provided. In this reading, the second conductive member may be left open.
The radiation image recording device according to the present invention is a radiation image recording device which realizes the above-stated radiation image recording method, comprising:
voltage application means which applies a DC voltage across the first electrode layer and the second electrode layer in the radiation solid-state detector, and
control voltage application means for applying, to the first conductive member, a control voltage to adjust the electric field formed between both electrode layers by a DC voltage applied by the voltage application means.
The radiation image recording device according to the present invention is a radiation image recording device which realizes the above-stated radiation image recording method, comprising image signal acquisition means which, by reading out the charges corresponding to the latent image charges stored in the charge storing section of the radiation solid-state detector through the first conductive member, provides an electric signal at a level corresponding to the quantity of the latent image charges.
When a detector with which the electrode of the second electrode layer and the electrode of the first conductive member are a stripe electrode, and the linear electrodes of the first conductive member are disposed so that they almost orthogonally intersect the linear electrodes of the second electrode layer is used, it is preferable that, by carrying out switchover so that only the linear electrode corresponding to the scanning position in scanning with the reading light is connected to the first electrode layer and the linear electrodes of the second electrode layer, the distributed capacitance which does not contribute to the signal reading be reduced.
The basic detector to which the present invention is to be applied may be any detector, provided that it is a detector with which electrodes are stacked so that they sandwich the photoconductive layer for recording and the photoconductive layer for reading, and it is preferable that the present invention be applied, particularly, to a detector (a static recorder) which has been proposed by the present applicant in the above-mentioned Japanese Patent Application No. 10 (1998)-232824, and Japanese Patent Application No. 10 (1998)-271374.
With the radiation solid-state detector according to the present invention, the first conductive member for outputting an electric signal at a level corresponding to the quantity of the latent image charges is provided between the second electrode layer and the first electrode layer, and therefore another capacitor can be formed between the charge storing section which is formed between the photoconductive layer for recording and the photoconductive layer for reading, and the first conductive member. The transported charges having a polarity opposite to that of the latent image charges stored in the charge storing section by recording can also be provided for the first conductive member by charge-recoupling in reading. The quantity of the transported charges distributed to the capacitor formed between the electrode of the second electrode layer and the charge storing section, through the photoconductive layer for reading, can be reduced to below that when this first conductive member is not provided. The quantity of the signal charges which can be taken out from the detector to the outside can be increased to improve the reading efficiency.
With the radiation image information reading method and device according to the present invention, the signal charges representing radiation image information are read out from the detector according to the present invention in which the radiation image information has been recorded, through the first conductive member, for providing an image signal at a level corresponding to the quantity of the latent image charges stored in the charge storing section. Therefore, more charges can be read out from the detector, which allows the reading efficiency to be increased, a larger signal to be obtained, and the S/N ratio for an image to be improved.
Providing the first conductive member will have practically no great effect on the thickness of the photoconductive layer for recording or the photoconductive layer for reading, which means that the response in reading will not be adversely affected, and as stated in Japanese Patent Application No. 10 (1998)-232824 and Japanese Patent Application No. 10 (1998)-271374, for example, by decreasing the sum of the thickness of the charge transporting layer and that of the photoconductive layer for reading to below the thickness of the photoconductive layer for recording, the response in reading can be enhanced. In other words, according to the present invention, the high speed response in reading can be maintained, while the reading efficiency can be increased to a level still higher than that obtained when a conventional detector is used.
Further, with the radiation image information reading method and device according to the present invention, a control voltage to adjust the electric field formed between the first electrode layer and the second electrode layer is applied to the first conductive member, which allows the signal taking-out efficiency and the reading response speed to be improved.
If the detector is that with which the second conductive member for causing the latent image charges to have the same potential is provided discretely in the charge storing section for each pixel for an image represented by the electrical signal, all the latent image charges for each pixel stored on the second conductive member can be caused to have the same potential, which allows the reading efficiency to be increased to a level higher than that obtained when the second conductive member is not provided. This is because the latent image charges are held at the same potential in the range of the second conductive member, therefore, the latent image charges around the pixel, which are generally difficult to read out, can be moved to the central portion of the second conductive member, i.e., the pixel central portion, as the reading progresses, so long as they are in the range of the second conductive member, which means that the latent image charges can be sufficiently discharged.
In addition, the pixel can be formed at the fixed location where the second conductive member is disposed, which allows the structure noise to be compensated for.
Further, if the size of the second conductive member is set at a value smaller than the pixel pitch, and disposed in the central portion of the pixel, the shape of the electric field formed in reading can be such that the electric field is attracted toward the second conductive member, therefore, the latent image charges can be stored, being concentrated on the pixel central portion, which means the sharpeness of the image can be improved.
If the second conductive member is provided, the latent image charges can be stored with no charge transporting layer or trap layer, which means, device formation can be performed more easily.
When the detector having a charge transporting layer or trap layer is provided with the second conductive member, the charge storing effect of such a layer can also be utilized. In other words, if the size of the second conductive member is set at a value smaller than the pixel pitch, and such a layer is not provided, a problem arises that the charges which have not been caught by the second conductive member cannot be stored as latent images, resulting in the quantity of the stored charges being decreased, although they contribute to the improvement in sharpness. Contrarily, when such a layer is provided, the charges are caused to be stored as latent image charges, so that the sharpeness can be improved without the quantity of the stored charges being reduced.
As with a detector, and a recording device and reading device as proposed in the above-mentioned Japanese Patent Application No. 10 (1998)-232824, etc. by the present applicant, the present invention is intended to provide compatibility between the high speed response in reading and the efficient signal charge taking out, and to offer a radiation solid-state detector which makes it possible to provide still higher performance than that of the device as stated in Japanese Patent Application No. 10 (1998)-232824, a method and device for recording of radiation image information in this detector, and a method and device for reading out the radiation image information from the detector in which the radiation image information has been recorded.